A. Technical Field
The present invention relates to a switch-mode regulator, and more particularly, to systems, devices and methods of using a switch-mode regulator to regulate an LED current in order to improve overall system efficacy and suppress power consumption of a dimmable light emitting diode (hereinafter, “LED”) illumination system. Such a switch-mode regulator may also be used in applications other than the LED illumination system.
B. Background of the Invention
Semiconductor-based solid-state lighting (SSL), until recently associated mainly with simple indicator lamps in electronics and toys, has become more common as SSL solutions continue to improve when compared to other lighting technologies. In particular, the enormous technology improvements have been achieved on light emitting diodes (LEDs) over the past years. LEDs have been available for various wavelengths, and suitable for white illumination. Lifetime of LEDs is also extended to more than 100 thousand hours, and can work up to many watts input power.
When compared to conventional lamps, LEDs are relatively smaller, require significant lower amount of power, and have a longer operating life. LEDs are normally connected in series as an LED string for use in lighting applications. Each power LED in the LED string used for illumination requires a nominal LED current in the range of 35-1400 mA, a forward voltage drop of 3V, and large manufacturing tolerances. The amount of power for LEDs to operate is much less than that of a typical halogen lamp. A halogen lamp may operate within a range of 20-50 Watts, while an LED at about 5-10 Watts is sufficient to provide a similar level of brightness. However, a need always exists to further enhance the energy efficiency of an LED illumination system.
FIG. 1A illustrates a standard LED illumination system 100 and power dissipation in such a system. The LED illumination system 100 comprises an LED driver 102 and an LED light module 104. The LED driver 102 is coupled to receive a DC supply voltage VSUP from a DC supply 106, and the voltage VSUP is further converted by the LED driver 102 to a drive voltage VT and an LED current 108. The LED light module 104 is driven by this drive voltage VT and the LED current 108. Illumination power PLED, i.e., brightness, of the LED diodes in the module 104 is directly associated with the LED current 108 that passes through the LED diodes.
In the LED illumination system 100, an input power PIN provided by the supply 106 is not fully converted to the illumination power of the LED light module 104. Both the LED driver 102 and the LED light module 104 dissipate some energy in the format of heat and invisible radiation during their respective process for voltage conversion and LED illumination. In particular, the LED driver 102 is typically based on a linear or switch-mode regulator. The linear regulator exhibits poor efficiency characteristics across its load range. The switch-mode regulator exhibits better efficiency than the linear regulator, but may still suffer from reduced efficiency when it is loaded below a maximum efficiency operating point due to inherent switching and quiescent bias losses within the switch-mode regulator.
The LED system efficacy ESYS refers to the overall illumination efficiency of the LED illumination system 100. Since any energy loss in the LED drive process has to be accounted, the LED system efficacy ESYS is a combination of the driver efficiency ηDRV and an efficacy ELED of the LEDs in the LED module 104. Hence, the LED system efficacy, the efficacy of the LEDs and the driver efficiency may be respectively represented as
            E      SYS        =                            η          DRV                ×                  E          LED                    =                        P          LED                          P          IN                                η      DRV        =                  P        OUT                    P        IN                        E      LED        =                  P        LED                    P        OUT            wherein POUT is the output power from the LED driver 102.
FIG. 1B illustrated a diagram 150 indicating two exemplary curves 152 and 154 of the efficacy ELED of the LEDs and the driver efficiency ηDRV in relevance to the LED current 108. The efficacy ELED of the LEDs and the driver efficiency ηDRV reach their peak levels ELOPT and ηDOPT at different LED currents ISMAX and IDMAX, respectively. In a typical LED illumination system 100, a preferred LED current IDMAX is determined in between IMIN and a maximum LED current IMAX such that the maximum driver efficiency ηDOPT is maintained between IMIN and IDMAX. Therefore, combined with DC dimming current through the LEDs, the maximum system efficacy ESYS is maximized across the entire dimming range IMIN to IMAX.
Various solutions are adopted to drive a dimmable LED illumination system. In one solution, the LED current 108 is generated as a pulse waveform having a frequency, magnitude and duty cycle that are modulated according to a dimming control. The magnitude of the LED current 108 alternates between zero and the maximum LED current IMAX during LED dimming operation. Thus, the LED driver 102 either is disabled or works at a driver efficiency associated with the maximum LED current IMAX, and however, the efficacy ηLED of the LEDs is compromised to work at a relatively low end of the efficacy curve 152 in such a solution. In another solution, the LED current 108 is generated as a direct current (DC) according to the dimming control. Although the LEDs function efficiently under a DC driver current, the LED driver 102 and its driver efficiency ηDRV is compromised in order to provide such a DC current. Therefore, both solutions cannot reach a preferred overall LED system efficacy upon receiving the varying dimming control.
Although it originates from the dimmable LED illumination system based on a switch-mode regulator, the above efficiency regulation issue commonly exists in a switch-mode regulator or power supply that generates an adjustable average output load current. A need exists to maintain preferred driver efficiency when the switch-mode regulator generates the adjustable average output load current.